Study on Transformation of Advanced Bainite Steel for Automobile after Continuous Cooling

Advanced bainite steel is an advanced high strength steel of the third generation. On the basis of ensuring automobile safety, it lightens the weight of the automobile body in order to reduce weight. Advanced bainite steel is mainly used in manufacturing reinforcing parts of automobile, such as pillars A and B, as well as the upper side beam. In this study, the experiment was conducted by adopting Gleeble-3800 thermal simulator and by combining with the methods of thermal expansion and metallographic hardness. The object of study was Advanced bainite steel, and the main objective was to test for its compressible deformation and the subsequent dynamic continuous transformation curve. Moreover, its hot rolling production process was simulated to study the rolling process parameters on the effect of microstructure. The experimental results showed that while the cooling rate was lower than 1°C/s, the microstructure of the experimental steel was mainly polygonal ferrite and pearlite. While the cooling rate was larger than 3°C/s, its hardness value sharply increased accordingly. While the cooling rate was larger than 10°C/s, martensite occurred in its microstructure. The microstructure mainly included reticular ferrite, bainite and martensite, and the hardness value increased at a greater speed. While the cooling rate was larger than 20°C/s and up to 50°C/s, lath martensite remained in its microstructure. While the cooling rate was at 50°C/s, the hardness value achieved the highest value at 490HV. The above analysis indicated that for Advanced bainite steel for automobile, a more suitable cooling speed should likely be controlled between 1~10°C/s. More significantly, the microstructure is mainly composed of ferrite, pearlite and bainite, and under the condition of the soft and hard phases being combined in a moderate proportion, there could be an obtainment of better matching for strength and toughness.